The protection of electronic circuit elements against polarity reversal, e.g., wrong connection to a battery, is of considerable importance, particularly in automotive electronics. In case of a reversal of supply voltage polarity, an entire integrated circuit may be destroyed, thus causing considerable damage. Circuits employing CMOS devices are particularly sensitive due to the sensitive nature of the thin insulated gates of the FETs.
To avoid the consequences of a polarity reversal, a diode is commonly incorporated within the integrated circuit to prevent reverse current flow which would otherwise damage the circuit. In CMOS circuits this has the disadvantage that a diode can only be integrated in connection with "buried-layer technology". This, however, involves additional technical complexity and thus entails increased expense, which should be avoided by all means.
Another problem with semiconductor devices, particularly with CMOS devices, is that they are prone to destruction from electrostratic discharge (ESD). It then becomes necessary to protect the operating circuit with ESD protection circuitry, which undesirably adds to overall circuit complexity and requires additional space.
U.S. Pat. No. 5,229,635 entitled "ESD Protection Circuit and Method for Power-Down Application" to Bessolo et al. discloses a circuit for providing ESD protection for an open-drain CMOS I/O buffer circuit. The protection circuit includes an NMOS transistor having its drain connected to the VDD power supply of the buffer circuit; and its bulk region connected to the VSS power bus. ESD protection is afforded by a parasitic lateral NPN bipolar transistor that is inherent in the NMOS transistor. However, polarity-reversal protection is not apparent with this circuit structure.
It is therefore an object of the present invention to provide polarity-reversal protection which can be achieved in conventional MOS technology without additional technical measures.
It is a further object of the present invention to provide a circuit that affords such polarity-reversal protection which simultaneously provides ESD protection with the same circuit structure.